The present invention is directed to a method of drying fiber or yarn wound on spools with said spools located in two separate vessels so that the drying operation alternates between the two vessels. In the drying procedure, the fiber or yarn is heated with an approximately moisture-saturated heated gas which serves as the heat carrier in the first part of the drying operation and, after the heating, a vacuum is established in the vessel for completing the drying. The steps of the drying procedure are carried out alternately in the two vessels until final drying is achieved. Further, the present invention is directed to the apparatus for carrying out the drying procedure.
Such a drying procedure, as well as the apparatus for performing the procedure is known, for example, German Patentshrift 21 07 696 corresponding to Great Britain Pat. No. 1,315,445. The drying procedure is effected by circulating a gas serving as a heat carrier which is first humidified during the heating process for heating the fiber or yarn to 30.degree. to 60.degree. C. To reduce the drying effect of the gas, the moistening or humidification of the gas is so intense that the partial steam pressure is approximately equal to the partial steam pressure of the gas boundary layer at the capillary surface of the goods being dried and the partial steam pressure is, in turn, dependent on the temperature of the goods being dried corresponding to the steam curve. Accordingly, an intensive heating of the textile goods is obtained and at the same time drying of the goods is avoided. Subsequently, the vessel containing the goods is evacuated causing the boiling point to be lowered with the moisture in the fiber or yarn being evaporated and evacuated or drawn off by a vacuum pump. The drying procedure alternating between heating with a moist hot vapor or gas and subsequently drying by means of a vacuum is repeated until the fiber or yarn is sufficiently dried.
The use of a moist hot gas or vapor for heating the fiber or yarn is an unconditional prerequisite for its safe treatment. When a hot dry gas is used the drying time is reduced, however, there is the problem of irreparable damage to the fiber or yarn because of overdrying.
Since the heating and vacuum drying steps must be performed alternately, as mentioned, the known drying apparatus employs a pair of vessels in which the drying procedure is carried in an alternating manner. The two vessels are connected to a humidifying heating device and to an evacuating drying device by means of reversing valves. As a result, the drying process can be carried out in the two vessels at the same time using a single arrangement of a heater, a blower or fan, and an evacuating device including a condenser, since when the blower and heater are connected to one of the vessels and other vessel is connected to the evacuating device.
In practice it has been noted that the drying procedure using a vacuum comes to a halt rather rapidly. This occurs when the moisture located at the outer surface of the fiber or yarn wound on the spool is evaporated. The moisture contained in the inner layers, spaced inwardly from the surface of the spool evaporates due to the reduction in the boiling point, however, the steam diffuses very slowly through the intermediate layers to the surface layer. Tests have shown that the drying procedure comes to a stop after about one-sixth of the time required for the entire drying cycle.